How You Play the Game

Knife

"What do you mean...your first time killing something?" Jack asked, chills running down his back. Mark glanced at him.
"I-I mean, how would you killing something affect my soul?" He asked nervously.
"Weeeeell, when I knocked you out and the entire timeline reset, I-I might've...maybe...rewrote some of it. Like, I found a weird string of numbers...I mean, it wasn't that difficult. I just...kinda...I can't really explain it, I just knew what to do and...so...I switched our roles. Um, you're the good guy and I'm the murderer, yknow? So, yeah. And since I'm...not used to murdering stuff, I guess it just affected me more strongly than you, so...the entire plot has just gone haywire by now. And, um, so that's not actually your soul you're holding there. Um, it's mine. So, please be gentle..." Mark explained nervously, twiddling his fingers.
((Mark essentially broke the entire fourth wall of the game and rewrote the basic coding of Undertale. Shh, pretend it's possible.))
"But...if this black one...is yours, why's it with me?" Jack looked down at the pulsing black heart. It might not even be his anymore, and he wasn't quite sure how to take that.
"Simple sleight of hand, Jackaboy. That's right, I'm cool. Uh...right. Wrong time for jokes, Mark... When we fell, I woke up first. That's when I made the exchange. Chara hadn't appeared yet. Nobody saw me."
"But...why would you do that?" Jack whimpered.
"What could this possibly achieve?!" He shouted, clutching the black heart harder than he meant to. Mark doubled over in pain, and Jack gasped and loosened his grip on the SOUL.
"I-I'm sorry!" He kneeled down, hurriedly offering Mark the black heart back. Mark hesitantly grasped it and took it back. Jack reached into Mark's shirt pocket and took out his own pure red heart. Jack froze for a moment.
"It's been a long time...since I've seen it look like this." Jack mumbled, gently cupping his clean SOUL. Mark smiled, glad he could do one thing right, at least. Jack glanced at him.
"But...I still acted the same...as before..." He mumbled in shock.
"Um. Yes. About that. See, I like you as you are and I didn't want to rewrite your entire being, so I kinda left it like it was...um, even with your memories gone, you've got that 'never truly over even after a true reset' thing going on, I guess, I dunno. So, you still kinda steered towards killing things? But, you never actually did any killing this timeline. Um, if you remember. You fought them, but...uh...didn't actually kill them, yknow." Mark explained as best he could.
"R-right."
"In fact, you even chose to restart from the save point after I killed Toriel! See, you're getting better!"
"What...do you mean, you killed Toriel?"
"Oh, I threw in a bunch of poisoned hits in while you weren't looking. Kind of a shame you restarted, honestly, but what can you do." Mark had unwittingly developed a darker tone to his voice, and it scared Jack.
"Actually, it kind of pisses me off. So, I'm just gonna get even here..."
Then, before anyone could stop him, he whipped a knife out of his pocket and threw it at Sans, impaling his skull.
"Oh." Sans said simply, gasping quietly. He shook his head at Mark.
"Looks like I'm sans of life." The squat skeleton grinned, before collapsing into a pile of ash.
"What...Mark, why?!" Jack wailed, running to where Sans had just been standing.
"Oh, come on. You know how it feels, don't you? You should remember now. I mean, you say you feel guilty now that I've rewritten the world but...it's so exhilarating, isn't it? Killing things, I mean. I get why you liked it. Back in the old days." Mark smiled slyly.
"This isn't...Mark, this isn't working out. I don't think this was a good idea. We-we need to go back..." Jack was scared. He didn't like seeing Mark like this. So vicious and malevolent without even trying. He waved his hand to pull up the window for TRUE RESET. But it didn't work.
"Oh~, looks like I've got all the power now~." Mark waved his hand down and the window appeared. He didn't press the button though, just stood there giggling like a maniac.
"Oh, that's hilarious." He closed the window.
"As IF I'm going to go back! There's an entire WORLD down here filled with things to kill!" Mark snickered.
"No...please. Don't!" Jack pleaded.
"Oh, how the tables have turned. I'm sick of trying to save you. You're worthless anyways. Couldn't even be bothered to kill that useless goat. I had to do it for you." Mark taunted. Jack sniffled, tears falling down his face.
"Is this how you felt? Mark. Is this how you felt when you saw me hurting all those people?" Jack asked the figure who used to be his best friend. Of course, no response. Mark wasn't there anymore, not really. Not his Mark. He might never come back. And it was Jack's fault. He should've stopped Chara a long time ago. He should've done something to save himself. Now, in an attempt to save him, Mark destroyed himself. Jack was alone.
"AAAAAAAAAAAAAAAAAAAAAAAAAAAA!" The underground was resounding with the echoes of a piercing shriek.
"Shut up! Shut up!" Mark screeched, covering his ears in pain, but it was easily drowned out by the force of Jack's despair.
"Huff...huff...." When Jack ran out of air, he took the knife from the pile of Sans's ashes. Slowly, he prepared to fight Mark. Standing up on unsteady legs, he glared angrily at him.
"Mark...I'm sorry." He said quietly, approaching him slowly.
"Oh, stop that. You know who you're talking to."
Jack's legs turned to jelly and he lost his balance, falling to his knees at the sound of the voice. The same voice that once rang in his head.
"C-c-c-Chara." He whispered. He could tell now. Looking closely, he could see Mark's chocolate brown eyes replaced by blood red pupils that pierced everything with their stare.
"Thaaaaat's right. Looks like your good old Markimoo is dead and gone, huh? Too bad. He actually held on a little longer than I thought he would, but...it doesn't matter. I'll just finish you off here, then go on to kill everybody else easily." Chara bragged, strutting about in the snow.
Stop.
Mark wouldn't want this. Jack knew that Mark wouldn't want this. Whatever the cost, whatever he had to do, even if he had to give up his life, Mark would try his best to keep everyone in the underground alive. He always had, he always will. He's that kind of selfless. But he couldn't do it now. It was up to Jack.
Jack stood back up slowly, his eyes alight with determination. He held the knife steady, breathing calmly. Mark would forgive him for this. He would understand, and Jack knew it.
Chara could sense the abrupt attitude change and shifted their story.
"H-hang on now, please don't, I don't think you really want to. I mean, I'm just borrowing your friend, you'd be hurting him if you stabbed me, see, and-"
"SHUT UP AND LET ME KILL YOU." Jack demanded angrily, swiping at Chara. They easily dodged it.
"Ugh. It's pieces of trash like you that make me want to destroy the world." Chara complained, picking Jack up easily. They held him by the throat and giggled.
"All I need to do is one little twist of the neck and I'm free from you. How convenient." Chara smiled viciously, but for some reason their body didn't move.
"Huh?" Chara exclaimed in confusion.
"I-i can't...uh..." Chara blinked a few times, before they shuddered and their eyes closed. A few moments later, their eyes opened. But they were a deep, emotional brown this time.
"Jack? Jack, why am I holding you like this? This isn't a reenactment of the Lion King, yknow." Mark made a little joke, as if just a moment ago he hadn't been possessed by a demonic spirit. He set Jack back down in the snow.
"Hey, you're not cold or anything, are you? You can have my sweater." Mark offered. Jack shook his head.
"Why didn't you kill me?" He asked quietly.
"...Why would I want to kill you? Where's this nonsense coming from? I'm never going to kill you. Never have, never will."
"I doubt your will is enough to stop Chara."
"Heh, good point. Maybe I picked up some guts or something. Maybe it's that mystical 'power of love' people talk about in tv shows. I know I can't hurt you, though. That's the basis of it. Even if I wanted to, which I don't, I will always stop myself. I'd be much quicker to protect you than to hurt you. Seriously, 18 resets and you haven't figured that out? Because it's just plain as day that you shouldn't hurt people you love." Mark stood up and smiled down at Jack, before shuddering again. His face went blank, and was then replaced by a cruel smile.
"Oh, that's just sickening. Too bad this love is unrequited, since you never seemed to have any qualms about killing him. It's just the fact that he made you restart before you could. But you've thought about it. I've seen your head. It's a very dark place there." Chara teased.
"But, at the same time...you must see something in him, or you wouldn't have hesitated going up against him." They giggled, snatching the knife away from Jack before he could react.
"W-what?"
"I know just what to do. See, I can hurt both of you at the same time~! Kill him and break you. Soooooo going to be worth the death, and I'll still come back if you reset! It's so simple! All I do...is..." Chara raised the blade to their neck and made a swift slicing motion.
A red arc of blood curved out and splattered onto the snow.
Chara's laughter was carried by the wind, and it sounded like the laugh of the grim reaper itself.

A/N: Sorry. I felt like being mean. HahahahahaHAHAHAHAHAHAHAHAHA hope you haven't gotten attached to these characters because oops. This may or may not be a really bad chapter, idk.